tendency is observed in Quercus robur [90], Betula pubescens [94], Acer
palmatum [97], Quercus crispula [97], Quercus acutissima [39, 73], and many
other tree species [95].
Second, young leaves appearing in spring include less chemicals than mature
leaves, and once matured, their contents are kept nearly constant or slightly
increased. Structural compounds (e.g., cellulose) in Quercus agrifolia leaves rapidly
increase with leaf age [92]. Total carbohydrate increases by 77% on the average in
mature leaves (Alnus incana, Tilia americana, Ulmus americana, Acer rubrum,
Betula papyrifera, Populus deltoides, Quercus velutina, Quercus macrocarpa,
Quercus alba, Carya glabra, and Juglans nigra), with sugar and starch increasing
by 67% and 62% on the average, respectively [95].
3.2
Secondary Metabolites
Seasonal changes in the secondary metabolites are somewhat complex. The seasonal
trends differ among phenolic groups and also tree species. The first general pattern is
that young leaves in spring include more secondary metabolites than mature leaves.
Total phenolic (and astringency) concentrations are higher in new leaves of Qeurcus
agrifolia [92] and Quercus acutissima [39, 73]. Phenolic concentrations in the
youngest leaves of Populus deltoides are three times those in the oldest leaves
[100]. Tannin concentrations of Leea glabra young leaves are also higher than that
of both medium-aged and old leaves [101]. The concentrations of cell wall-bound
proanthocyanidins, gallotannins, and flavonoid glycosides decline after an initial
increase in young leaves of Betula pubescens [94]. Total phenolic and hydrolyzable
tannin contents in Acer saccharum leaves tend to decline from spring to summer but
no further decline after summer [102]. Young Quercus robur leaves are much richer
in the dominant phenolic hydrolyzable tannins and flavonoid glycosides than old
leaves, although the opposite pattern is observed for a minor phenolic proanthocyanidins [103].
The second general pattern is that the secondary metabolites increase with leaf
growing from spring to summer and keep high during summer and autumn. Total
phenolic content of Moringa oleifera leaves is lowest in the newly opened leaves and
increases gradually with the maturation of the leaves [104]. It peaks by the summer
and then remains relatively constant through autumn in Acer saccharum [96] and in
Quercus robur [91]. Pirvu et al. [105] report an increase of total phenolics in Fagus
sylvatica leaves, with overall maximum values in September, and Sati et al. [106]
also report a similar increase in Gingko biloba leaves, higher in the fall than in
spring. Concentrations of condensed tannins gradually increase as the leaves mature
[92]. Condensed tannin levels are also lowest in spring and increase throughout the
growing season for Quercus robur [107] and Quercus velutina [108]. The tannin
content increases during summer in Acer palmatum and Quercus crispula [97], and
soluble proanthocyanidins content increases through growing season in Betula
pubescens [94]. In Acer saccharum leaves, condensed tannin contents cannot be
detected in the spring samples but reach a plateau through summer [102]. Phenolic
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palmatum [97], Quercus crispula [97], Quercus acutissima [39, 73], and many
other tree species [95].
Second, young leaves appearing in spring include less chemicals than mature
leaves, and once matured, their contents are kept nearly constant or slightly
increased. Structural compounds (e.g., cellulose) in Quercus agrifolia leaves rapidly
increase with leaf age [92]. Total carbohydrate increases by 77% on the average in
mature leaves (Alnus incana, Tilia americana, Ulmus americana, Acer rubrum,
Betula papyrifera, Populus deltoides, Quercus velutina, Quercus macrocarpa,
Quercus alba, Carya glabra, and Juglans nigra), with sugar and starch increasing
by 67% and 62% on the average, respectively [95].
3.2
Secondary Metabolites
Seasonal changes in the secondary metabolites are somewhat complex. The seasonal
trends differ among phenolic groups and also tree species. The first general pattern is
that young leaves in spring include more secondary metabolites than mature leaves.
Total phenolic (and astringency) concentrations are higher in new leaves of Qeurcus
agrifolia [92] and Quercus acutissima [39, 73]. Phenolic concentrations in the
youngest leaves of Populus deltoides are three times those in the oldest leaves
[100]. Tannin concentrations of Leea glabra young leaves are also higher than that
of both medium-aged and old leaves [101]. The concentrations of cell wall-bound
proanthocyanidins, gallotannins, and flavonoid glycosides decline after an initial
increase in young leaves of Betula pubescens [94]. Total phenolic and hydrolyzable
tannin contents in Acer saccharum leaves tend to decline from spring to summer but
no further decline after summer [102]. Young Quercus robur leaves are much richer
in the dominant phenolic hydrolyzable tannins and flavonoid glycosides than old
leaves, although the opposite pattern is observed for a minor phenolic proanthocyanidins [103].
The second general pattern is that the secondary metabolites increase with leaf
growing from spring to summer and keep high during summer and autumn. Total
phenolic content of Moringa oleifera leaves is lowest in the newly opened leaves and
increases gradually with the maturation of the leaves [104]. It peaks by the summer
and then remains relatively constant through autumn in Acer saccharum [96] and in
Quercus robur [91]. Pirvu et al. [105] report an increase of total phenolics in Fagus
sylvatica leaves, with overall maximum values in September, and Sati et al. [106]
also report a similar increase in Gingko biloba leaves, higher in the fall than in
spring. Concentrations of condensed tannins gradually increase as the leaves mature
[92]. Condensed tannin levels are also lowest in spring and increase throughout the
growing season for Quercus robur [107] and Quercus velutina [108]. The tannin
content increases during summer in Acer palmatum and Quercus crispula [97], and
soluble proanthocyanidins content increases through growing season in Betula
pubescens [94]. In Acer saccharum leaves, condensed tannin contents cannot be
detected in the spring samples but reach a plateau through summer [102]. Phenolic
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